Distribution of small seasonal reservoirs in semi-arid regions and associated evaporative losses

Abstract

To support the increasing demand of a growing population for freshwater, small-sized (<0.1 km 2 ) water reservoirs are necessary in areas with limited infrastructure, especially in water-stressed regions having seasonal and variable precipitation. Seasonal storage in small reservoirs is often overlooked in present inventories. Accordingly, we assessed the current state in semi-arid regions using highly resolved Sentinel-2 satellite imagery. Globally, about 3 million small reservoirs are in operation in semi-arid regions having a total water surface area of 17 800 km 2 and seasonal storage of 37 km 3 , supporting 15% of the world's population in semi-arid regions. Estimated upper-bound of evaporative losses represent 38% (14 km 3 ) of storage during dry season. The study estimates the evaporation suppression using floating covers and lays foundation to assess the feasibility of this technical solution to increase water savings. Significance and originality statementThis study provides new insights into the distribution of seasonal freshwater storage in water stressed regions that support large rural population. We provide the first baseline dataset of this distributed freshwater resource to better assess the sustainability of communities in semi-arid regions with chronic water shortages. Water storage in semi-arid regions is subject to potential evaporative losses of nearly 40% of the stored water. We analyzed conditions where evaporative losses could be significantly reduced using floating covers, a simple and scalable solution suitable for regions with underdeveloped economies. The study as well ranks regions with highest potential benefits from evaporation suppression measures.

Evaporation Suppression From Small Reservoirs Using Floating Covers—Field Study and Modeling

“…In an ongoing study, the effects of floating covers on biological parameters is analyzed to proof that the reduction of energy storage and sunlight penetration does not have negative effects on the water quality (but rather positive consequences; oxygen concentrations are normal or over the saturation level and blooms of cyanobacteria are reduced). Finally, Mady et al (2020) analyzed the economic aspects of floating elements (comparing potential water saving with alternative water sources) and concluded that floating covers are also a cost-efficient solution for semiarid regions.…”

“…The higher price of the spheres (about the triple of the price of disks per m 2 ) is compensated by (i) the higher durability of the material, (ii) better suppression efficiency and (iii) less maintenance (no need to collect elements after storm) so we conclude that both cover types have similar overall performance for a reasonable long lifetime. Mady et al (2020) analyzed the physical and economic aspects of floating disks in semiarid areas (comparing potential water saving in dry season from spatially distributed small reservoirs with alternative water sources in the region). The calculations were driven by the modeling results from Aminzadeh et al…”

“…Estimates suggest that in hot periods evaporation losses from water reservoirs range from 10% in Spain (Martínez Álvarez et al, 2008) to 50% in Australia (Craig 2005;Rost et al, 2008). Regionally resolved analyses by Mady et al (2020) suggest that globally 30% of the stored volume in small reservoirs could be lost to evaporation during the dry season (in semiarid regions).Various mitigation techniques have been proposed for reducing evaporation losses, differing in their suppression efficiency and construction and maintenance costs (…”

Reducing Evaporation From Water Reservoirs Using Floating Lattice Structures

“…(2018), which assumed Styrofoam covers with a lifetime of 10 years and cover cost of 5 $ per m 2 of water surface and annual maintenance cost of 0.1 $ per m 2 of covered surface, Mady et al. (2020) concluded that floating covers are a cost‐efficient solution for almost all the semiarid regions. Similar approaches should be applied to estimate the economic viability of lattice‐like covers.…”

Comment on essd-2021-58

“…Alternatively, since we imposed no usage restrictions on our spatial features (geometric dam points and reservoir polygons), users are free to integrate them with other datasets and tools, such as remote sensing observations and modelling, to acquire the needed attributes, particularly those not yet documented in ICOLD WRD. Acquisition methods have been exemplified for at least the following attributes: reservoir hypsometry and bathymetry (Li et al, 2020;Yigzaw et al, 2018), surface evaporation loss (Mady et al, 2020;Zhan et al, 2019;Zhao and Gao, 2019a), operation rules (Shin et al, 2019;Yassin et al, 2019), completion years (Zhang et al, 2019), storage capacities (Liu et al, 2020), and the changes in water area (Pekel et al, 2016;Yao et al, 2019;Zhao and Gao, 2019b), level (Cretaux et al, 2011;Schwatke et al, 2015), and storage or volume (Busker et al, 2019;Cretaux et al, 2016;Gao et al, 2012;Zhang et al, 2014).…”